Multi-layer piezoelectric actuators constituted from piezoelectric layers and internal electrodes stacked alternately one on another have been known. The multi-layer piezoelectric actuators can be divided into two categories: fired-at-once type and stacked type which has such a constitution as piezoelectric porcelain made of a piezoelectric material and internal electrodes having the form of sheet are stacked one on another alternately. When the requirements to reduce the operating voltage and the manufacturing cost are taken into consideration, the multi-layer piezoelectric actuator of fired-at-once type is more advantageous for the reason of smaller layer thickness and higher durability.
FIG. 7 shows a multi-layer piezoelectric element of the prior art disclosed in Patent Document 1, which is constituted from a stack 20 and external electrodes 54 formed on a pair of opposing side faces thereof. The stack 20 is formed by stacking piezoelectric layers 51 and internal electrodes 52 alternately one on another. The internal electrodes 52 are not formed over the entire principal surfaces of the piezoelectric layers 51, but are formed in a so-called partial electrode structure. In the stack of the partial electrode structure, the internal electrodes 52 are stacked in a staggered manner so as to be exposed on the side face of the stack 20 alternately at the left in one layer and then at the right in the next layer. The external electrodes 54 are formed so as to be connected to the internal electrode 52 that is exposed on either of a pair of opposing side faces of the stack 20 in every other layer.
Inactive layers 62 are stacked on both end faces of the stack 20 in the direction of stacking. The inactive layer 62 is also called the protective layer, and normally does not include the electrode 51. This constitution involves such a problem that a portion including the internal electrode layer 52 and the inactive layer 62 shrink to different degrees during firing, thus resulting in stress and/or cracks generated therein. To counter this problem, as shown in FIG. 8, Patent Document 3 discloses means for preventing cracks from occurring after firing, by stacking an electrode layer 61 that is similar to an active layer 63 on the inactive layer 62. A portion including the internal electrode layer 52 that is connected to the external electrode is called the active layer since it expands and contracts in response to the application of voltage.
The multi-layer piezoelectric element having the constitution described above is manufactured as follows. First, a paste for the internal electrode is printed in a predetermined pattern of the electrode structure on a plurality of ceramic green sheets that include the substance used for making the piezoelectric material 51, and are stacked and fired so as to form the stack 20. Then the external electrodes 54 are formed on a pair of side faces of the stack 20 thereby to make the multi-layer piezoelectric element (refer to, for example, Patent Document 1).
The internal electrode 52 is formed from an alloy of silver and palladium and, in order to fire the piezoelectric layers 51 and the internal electrodes 52 at the same time, composition of metals included in the internal electrode 52 has been set typically to 70% by weight of silver and 30% by weight of palladium (refer to, for example, Patent Document 2).
The reason for using the internal electrode 52 made of a metal compound that includes silver-palladium alloy instead of the internal electrode 52 having composition constituted from pure silver is that, when a voltage is applied between a pair of opposing internal electrodes 52, the composition constituted from pure silver suffers from the so-called silver migration in which silver atoms migrate from the positive electrode to the negative electrode of the pair of internal electrodes 52 along the device surface. Silver migration occurs conspicuously particularly in an atmosphere of high temperature and high humidity.
When the multi-layer piezoelectric element of the prior art is used as a piezoelectric actuator, lead wires (not shown) are secured onto the external electrodes 54 by soldering, and a predetermined voltage is applied across the external electrodes 54 to operate the device. In recent years, it is a common practice to operate the multi-layer piezoelectric element continuously over a long period of time, since it is required to achieve a large amount of displacement from a compact multi-layer piezoelectric element under a higher pressure.    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 61-133715    Patent Document 2: Japanese Unexamined Utility Model Publication (Kokai) No. 1-130568    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 9-270540